Constant Quantizer
Quantizer/Bitrate: This setting is basically a trade-off between visual quality and filesize. If you are using bitrate mode, then you simply set the bitrate that you want to encode your video at. Higher bitrates increase the size of your output file, and you can see the estimated filesize as you modify the bitrate value. If you are using either constant quantizer or constant quality mode, then this is where you select the quantizer to use. Valid values are from 0-51, with lower values giving better quality, but larger filesizes. A quantizer of 18 is generally considered the lowest that you should select, as it's unlikely that you will see much visual improvement by going any lower. For those of you familiar with Xvid, a quantizer of 18 here is equivalent to Xvid's quantizer 2. The default quantizer value is 20, which gives a rather decent tradeoff between quality and size.
http://www.animemusicvideos.org/guides/avtechbeta/zarx264gui.html
Constant Quality maps (not sure of the formula) to a specific CRF. CRF, like CQP (Constant QP/Quantizer), ranges from 0 to 51.
Constant Rate-Factor (CRF) mode is similar to Constant Quantizer mode, but uses higher quantizers on more complex frames and lower quantizers on simpler frames. Overall, the result is better SSIM and PSNR than one would get with constant quantizer mode. The formula used is very similar to the one used for two-pass ratecontrol, except that since it isn't trying to achieve a specific bitrate, it can do the same in only one pass.
However, it is definitely not at all a "constant PSNR" or "constant SSIM" mode--rather, its a "like CQP, but better" mode.